Bombadil's brooch

[Raynor]

I don't think that the barrows enjoyed a relative inaccessibility. Hammond and Scull, in their comments say that:

Originally Posted by The Old Forest, "the Barrow-downs", page 122, LotR Reader's Companion

In Nomenclature, Tolkien describes the Barrow-downs as "low, treeless hills on which there were many "barrows", sc. tumuli and other prehistoric grave-mounds".

Then again, I have zero military experience, so you may be right. Hammond quotes Shippey saying that Tolkien must have seen barrows in person, including the famous Wayland's Smithy, which was fifteeen miles from Tolkien's study.
I also think that the servants of Sauron/Witch-king would have wasted no time in searching the mounds themselves, as a blasphemy, as I believe they had little if any fear of necromancy (and they might have also known that necromancy was a no-no for their enemies). Imo, the dunedain weren't hiding in the barrows, nor would they have used them as defensive positions (I believe in Vietnam one would just smoke them out).